Wireless communication networks, such as cellular networks, operate by sharing resources among the mobile terminals operating in the communication network. As part of the sharing process, resources relating to which channels, codes, etc., are allocated by one or more controlling devices within the system. Certain types of wireless communication networks, e.g., orthogonal frequency division multiplexed (“OFDM”) networks, are used to support cell-based high speed services such as those under the IEEE 802.16 standards. The IEEE 802.16 standards are often referred to as WiMAX or less commonly as WirelessMAN or the Air Interface Standard. Another emerging standard that has not yet been ratified, but appears likely to use the OFDM modulation scheme is referred to as Long Term Evolution (LTE). Other networking protocols may also use OFDM.
OFDM technology uses a channelized approach and divides a wireless communication channel into many sub-channels which can be used by multiple mobile terminals at the same time. These sub-channels and hence the mobile terminals can be subject to interference from adjacent cells because neighbouring base stations can use the same frequency blocks.
As such, a method is needed to adjust the output power of the base stations to a value which enables mobile terminals at the cell edge to still communicate at an acceptable rate while not creating too much interference in neighbouring cells. A method, described below, has been proposed to reduce interference caused by neighbouring base stations using the same frequency blocks and thereby help to increase mobile terminal throughput and overall network capacity.
For cell edge mobile terminals, i.e., mobile terminals near the edge of a cell, a base station has to use more transmission power in order to reach them. Cell centre mobile terminals, i.e., mobile terminals near the base station, require much less transmission power to receive the signal. Because known mobile terminals only transmit and receive on some but not all sub-channels of the frequency band, transmission power of sub-channels used by mobile terminals close to a base station can be lower than the transmission power of sub-channels used by mobile terminals at the cell edge. In practice, the reduced transmission power for sub-channels used by mobile terminals close to a base station creates less interference for mobile terminals close to other base stations.
It is known that a combination of high and low power sub-channels can be used to increase the overall coverage of the network compared to networks which use the same transmission power for all sub-channels. Base stations can be organized in a way to use the same set of sub-channels to serve subscribers close to them with a low transmission power. The remainder of the sub-channels are used with a higher transmission power and can be used by both distant and close subscribers. To minimize interference of high power sub-channels for clients of neighbouring base stations the cells are further organized in a way that two adjacent cells do not use the same high power sub-channels. As such, cell edge mobile terminals can be scheduled on the high power sub-channels that are not used or are used with lower transmit power by the neighbouring sectors. This approach is known as fractional frequency re-use (“FFR”) as all base stations use the same frequency band with different power level restriction on different sub-channels.
Known fractional frequency reuse (FFR) schemes include hard and soft FFR schemes. Reuse, refers to the quantity of cells or sectors serviced by a base station. In such existing FFR solutions, however, the reuse factor, i.e., pattern, is fixed. In other words, the reuse arrangement of high power sub-channels and low power sub-channels within a cell can not be changed, even though the conditions within the cell have changed, either for the better or worse. Thus, there will always be some impact on throughput in known systems even when there are no coverage problems. Fixed FFR schemes, in addition, require frequency planning. In soft reuse, a portion of the tones are used with lower power, whereas, in hard reuse, a portion of the tones are not used at all.
United States Patent Application Number 2009/0061778, the contents of which are incorporated by reference herein, discloses a method and system for FFR whereby a base station serving a cell or sector sends a power level adjustment indicator over the backhaul network to a list of neighbouring base stations when it detects a coverage problem. The neighbour list can be either pre-configured or can be created dynamically by including the most dominant interferers of each of the mobile terminals being served. This approach provides a way of slowly adapting power of interfering base stations, and can be used to adapt many base stations since the serving base station can send power level adjustment commands to a number of neighbouring base stations over the backhaul network.
A need exists for new FFR methods and systems that allow for the reuse of frequencies based on the detection of coverage problems within the cell or sector.